The invention concerns a laser cutting machine with a laser generator as well as with a laser cutting head, which can be moved relative to the laser generator and/or relative to a workpiece in a plane essentially parallel to the workpiece by means of a drive controlled by a numerical control and has a focussing lens for the laser beam as well as an adjusting device for adjusting the focal position of the laser beam by shifting the focus with respect to the laser cutting head essentially perpendicularly to the workpiece.
On laser cutting machines with a so-called "flying lens," the laser cutting head with the focussing lens for the laser beam is moved relative to the workpiece which is fixed in place during cutting; because of the constantly changing position of the laser cutting head, the laser beam has to cover constantly changing path lengths from the laser generator to the focussing lens on the laser cutting head. Among other things, the laser beam path length is decisive for the position of the laser beam focus created by means of the focussing lens which direct the beam perpendicularly to the workpiece to be cut. However, in order to be able to ensure a uniform cutting quality, respectively uniform cutting data characteristics, during the entire working process, it must be made sure that the focal position also does not change, or at most barely changes, even in the case of a change in the position of the laser cutting head and the change in the laser beam path length.
The entire laser cutting head is raised or lowered in laser cutting machines in the known way in relation to the laser beam length in order to obtain an unchanging focal length perpendicular to the workpiece over the entire range of motion of the laser cutting head parallel to the workpiece to be cut. The position of the focus in this case remains unchanged relative to the laser cutting head.
An adjusting device for the optical adjustment of the focal position of the beam, developed for a generic laser cutting machine, is in the form of adaptive spherical surface mirrors for the laser beam, which are arranged ahead of the focussing lens of the laser cutting head in the direction of passage of the laser beam. This serves for adjusting the focal position by shifting the focus with respect to the laser cutting head. The curvature of the mirror surfaces, and thus their focal length, can be varied by having changing pressures act upon the surfaces of the surface mirrors opposite the reflecting surfaces. A change in the divergence or convergence, of the laser beam, which is reflected from the surface mirror arranged directly ahead of the focussing lens onto the latter, results from a change in the mirror curvature. Now if the laser beam geometry changes as a result of a position change of the laser cutting head and a change in the beam path length connected with this, then this change in geometry can be compensated by varying the mirror curvature. As a consequence of this, the position of the focus created by the focussing lens perpendicular to the workpiece remains unchanged and independent of the length of the laser beam path.
In the known test arrangement only the described general functional principle for optical adjustment of the focal position of the laser beam is realized by means of adaptive mirrors.
The task of the invention now is to prepare a laser cutting machine, capable of functioning under workshop conditions and suited for automatic operation, which permits a functionally sure optical adjustment of the focus position.